Polythioaminals are a class of polymers with a variety of uses, including drug delivery. Some polythioaminals are polymers that have the general structure:
where R1 and R2 are organic or hetero-organic species. It has been shown that polythioaminals having the above structure may be synthesized by reacting an N-substituted hexahydrotriazine with a dithiol, as follows:
Subsequent reactions may replace the hydrogen atoms at the end of the thioaminal polymer with the X and Z groups above.
Whether a polythioaminal can achieve a high molecular weight during polymerization is significantly affected by stoichiometric ratios of starting materials, in view of the Carother's equation. The Carother's equation states that the degree of polymerization of a monomer (into a polymer) is equal to 1/(1−p), where p is the extent of conversion of a monomer. Small changes in the stoichiometry of one of the polymerization reactants may significantly affect the molecular weight of a synthesized polymer.
Furthermore, the reaction shown above generates a byproduct amine R2—NH2. As the reaction proceeds, concentration of the byproduct amine grows to an extent that limits progress of the reaction from achieving high degrees of polymerization. If the byproduct amine is not removed, molecular weight of the polymer generally does not grow above about 5,000 Daltons. Amine scavengers may be added to the polymerization reaction to capture R2—NH2 species. If the reaction is performed at elevated temperature, volatile byproduct amines may be vaporized to drive the reaction to higher molecular weight. However, some thioaminal polymers tend to decompose at temperatures above about 200° C., so use of such methods to increase molecular weight is limited.
Furthermore, growth of polythioaminals on substrate surfaces via two monomeric starting materials (e.g., dithiols and hexahydrotriazines) generally yields polythioaminals of less than about 5,000 Daltons.
Broadly applicable methods of increasing molecular weight of polythioaminals are needed.